The objectives of this research are to define the structures of tyrosine transfer RNAs in mammalian tissues and transformed cells and to characterize the enzymes responsible for the biosynthesis of pseudouridine in eukaryotes. Structural analyses will involve labelling RNAs with radioactive tracers, enzymatic or chemical degradation of the molecules, fractionation of products by gel electrophoresis, and identification of individual residues by chromatography. The enzymology of psi biosynthesis will consist of purification of the enzyme, characterization of its activity on defined substrates by sequence analysis, examination of its interaction with various substrate and non-substrate nucleic acids, and a systematic search for new substrates and additional psi synthetases.